1. Field of the Invention
This invention relates to an actuator, a method of manufacturing the actuator and a circuit breaker employing the actuator.
2. Description of the Background Art
Conventionally, permanent magnet actuators have been used in circuit breakers as disclosed in German Patent Publication No. DE 4304921 C1, for example. FIG. 28 is a diagram showing the construction of a circuit breaker 2 employing conventional actuators 1. Each of these actuators 1 is used to open and close contacts 4 which are arranged face to face with each other in a vacuum valve 3 of the circuit breaker 2, for example, by driving one of the contacts 4 in linear motion. Each actuator 1 includes a generally square-shaped yoke and a parallelepiped-shaped armature accommodated in an inner space of the yoke. The yoke has upper, lower, left-hand and right-hand yoke portions forming four sides of the square shape. Projecting inwards from central parts of the left-hand and right-hand yoke portions are magnetic poles which are situated on opposite sides at a specific distance from each other.
The armature is located between the opposing magnetic poles. On both side of the armature, there are provided plates which are supported movably up and down by bearings. The armature is sandwiched between these plates and screwed thereto. With this arrangement, the armature is supported movably up and down by means of the bearings in the inner space of the yoke. Permanent magnets are affixed to the individual magnetic poles in a manner that narrow gaps are created between the armature and the permanent magnets. The armature is held at a first position where the armature is attracted to the upper yoke portion and at a second position where the armature is attracted to the lower yoke portion by a magnetic force exerted by the permanent magnets.
To move the armature from one bistable position to the other, and vice versa, there is provided a pair of generally square-shaped exciting coils having square-shaped inside surfaces in the inner space of the yoke. As the armature is driven between the first and second bistable positions, it travels not only between the two opposing magnetic poles but also along the square-shaped inside surfaces of the exciting coils. When one of the exciting coils is excited, it produces an electromagnetic driving force which cancels out the magnetic force exerted by the permanent magnets at the first bistable position and attracts the armature to the second bistable position, causing the armature to move thereto.
When the other exciting coil is excited, it produces an electromagnetic driving force which cancels out the magnetic force exerted by the permanent magnets at the second bistable position and attracts the armature to the first bistable position, causing the armature to move thereto. As the armature is driven between the two bistable positions in this fashion, the movable contact in the vacuum valve 3 connected to the armature via the plates moves up and down, thereby opening and closing the contacts 4 in each vacuum valve 3.
In the conventional actuator 1 thus constructed, the armature moves up and down, controlled by currents flowed through the two exciting coils. Although it is desirable that the armature move while maintaining narrow gaps between the armature and the magnetic poles, and between the armature and the inside surfaces of the exciting coils, the armature could occasionally move in sliding contact with the permanent magnets or exciting coils due to manufacturing errors, for instance. In particular, if the armature moves in sliding contact with the permanent magnets, the permanent magnets wear and produce ferromagnetic powder. Should this ferromagnetic powder stay in the narrow gaps, it could prevent smooth movement of the armature, leading to a deterioration in reliability of operation of the actuator 1.
Furthermore, if the exciting coils are not securely fastened to the yoke, the exciting coils might be displaced due to shocks caused by movement of the armature or makebreak action of the vacuum valve 3, preventing smooth movement of the armature. To cause the armature to move up and down while maintaining narrow gaps between the armature and the magnetic poles, and between the armature and the inside surfaces of the exciting coils, it is desirable to support the armature with a pair of bearings provided at both ends of the armature to support it movably up and down. To achieve this, it is necessary to locate two bearings on a common axis along the moving direction of the armature as much as possible.